Temperature-Dependent s± ⇆ s++ Transitions in the Multiband Model for Fe-Based Superconductors with Impurities

We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the s(+/-) state with sign-changing gaps. Scattering by nonmagnetic impurities leads to the change of the sign of the smaller gap, resulting in a transition from the s(+/-) to the s(++) state with the sign-preserving gaps. We show here that the transition is temperature-dependent. Thus, there is a line of s(+/-) -> s(++) transition in the temperature-disorder phase diagram. There is a narrow range of impurity scattering rates, where the disorder-induced s(+/-) -> s(++) transition occurs at low temperatures, but then the low-temperature s(++) state transforms back to the s(+/-) state at higher temperatures. With increasing impurity scattering rate, the temperature of such s(++) -> s(+/-) transition shifts to the critical temperature T-c, and only the s(++) state is left for higher amounts of disorder.